(a) Field of the Invention
The present invention relates to a method of manufacturing an electronic component device. More specifically, it relates to a method of manufacturing an electronic component device including an electronic component such as a semiconductor element mounted on a board, and a heat-spreading component which dissipates to the air, heat generated from the electronic component, and which is coupled via a thermal interface material to the electronic component.
(b) Description of the Related Art
A semiconductor element (chip) used in a microprocessor unit (MPU) and the like is electrically connected to and fixed on a wiring board (package) (for example, by flip chip mounting). The semiconductor element is brought to a high temperature state during its operation. Accordingly, unless the semiconductor element is forced to be cooled down, the semiconductor element cannot demonstrate its intended performance, or may be broken down in some cases.
For this reason, a heat-spreading component (for example, a heat spreader made of metal) for dissipating heat generated from a semiconductor element, to the air, is disposed on the semiconductor element. The heat-spreading component ensures a passage through which heat generated from the semiconductor element is dissipated to the outside. In this situation, the semiconductor element is thermally coupled to the heat spreader with a material called a thermal interface material (TIM) interposed therebetween. Specifically, the TIM interposed therebetween fills asperities of both the semiconductor element and the heat spreader to reduce the thermal contact resistance therebetween, and thus heat is smoothly transferred from the semiconductor element to the heat spreader.
Examples of such a thermal interface material (TIM) include one obtained by forming a high thermo-conductive substance, such as indium, silicone (or hydrocarbon) grease, metal filler, graphite or the like, into a sheet shape with a resin binder. In the case of indium, an indium sheet is sandwiched between a heat spreader and a semiconductor element. Then, using a resin material which is cured at the melting point of indium or lower, the heat spreader is temporarily fixed to a package on which the semiconductor element is mounted. Thereafter, reflowing is performed on the assembly at the melting point of indium or higher. While the thickness of indium is controlled, the heat spreader is fixedly bonded to the semiconductor element.
In this process, where the thickness of indium (TIM) is too thick, the TIM has a high thermal resistance. On the contrary, where the thickness is too thin, the TIM cannot flexibly follow a deformation of the semiconductor element occurring due to the heat (warpage of the semiconductor element occurring due to the heat generated during the operation), and accordingly cannot demonstrate its properties sufficiently.
Moreover, indium is relatively expensive, and is a rare metal. Accordingly, the stable supply of future is concerned. Furthermore, since heat treatment is necessary in the reflowing and the like, there is also another concern in quality that a void is produced during the reflowing in addition to the problem of the complicated manufacturing steps by the reflowing.
For this reason, development of an alternative TIM has been desired. Such a TIM is desired to have stable properties and a high thermal conductivity equivalent to or higher than that of indium. As one of the alternative TIMs, development of a TIM using carbon nanotubes is in progress. This is a method in which carbon nanotubes are arranged in a direction of heat conduction and formed into a sheet shape with a resin. The carbon nanotubes arranged in the direction of heat conduction are fixed within the resin, and tip ends of the carbon nanotubes are in contact with a heat spreader and a semiconductor element. The carbon nanotubes are excellent in thermal conductivity and very high in bending strength (i.e., having an elasticity like a spring). Accordingly, the carbon nanotubes are expected to be capable of following a deformation (warpage) of the semiconductor element by the heat as described above.
An example of a technique related to the above-mentioned prior art is described in Japanese unexamined Patent Publication (Kokai) 2003-264261. This document discloses a thermal coupling sheet interposed between a heat-spreading fin and a heat-spreading plate serving as a cooling surface of a power semiconductor module. The thermal coupling sheet is made of a resin and the heat transferability is increased by a filler mixed therewith. As the resin constituting the thermal coupling sheet, used is a hot-melt resin which melts at a temperature at the time when heat is generated by feeding a current through the power semiconductor module.
In the prior art as mentioned above, the thermal interface material (TIM) using carbon nanotubes is progressively developed as the TIM interposed between an electronic component (semiconductor element) and a heat-spreading component (heat spreader). The carbon nanotubes have a high thermal conductivity equivalent to or higher than that of indium, and also have a very high bending strength. However, this method poses the problems such as described below, because the carbon nanotubes arranged in the direction of heat conduction are fixed within the resin and the tip ends thereof are in contact with the heat spreader and the like.
Specifically, the tip ends of the carbon nanotubes do not always surely come into contact with the surfaces of the heat spreader and the like, depending on the amount of the resin filled between the semiconductor element and the heat spreader, and/or the amount of pressing performed together with heating when the heat spreader is fixed to a package (on which the semiconductor element is mounted). If the tip ends of the carbon nanotubes are not in contact with the surfaces of the heat spreader and the like, the resin is then present in the non-contacting portion, and the thermal resistance in this portion is significantly increased. On the other hand, if the amount of pressing is increased and the carbon nanotubes are excessively pressed, the carbon nanotubes are laid down. As a result, the bending strength (elasticity) which the carbon nanotubes originally have cannot be fully utilized, and thus the carbon nanotubes may not flexibly follow a warpage (deformation) occurring during the heat generation by the semiconductor element.
FIGS. 5A and 5B schematically show such a state. As shown in FIG. 5A, carbon nanotubes 1 arranged in a direction of heat conduction (vertically in the illustrated example) are fixed within a resin layer 2. The carbon nanotubes 1 differ in length from one another. Meanwhile, a surface (lower surface in the illustrated example) of a heat spreader 3 with which the tip ends of the carbon nanotubes 1 are to come into contact has a microscopic asperity as illustrated. The degree of the asperity is small relative to the variation in length among the carbon nanotubes 1. For this reason, the tip ends of shorter carbon nanotubes 1 cannot reach the surface of the heat spreader 3, and the resin layer 2 is present in portions denoted by reference numeral 4 and surrounded by broken lines in the drawing. Accordingly, the thermal resistance in the portions is relatively increased. Moreover, even a carbon nanotube 1 which has a length enough to reach the heat spreader 3 is likely to have an extremely thin resin layer 2 interposed between the tip end thereof and the surface of the heat spreader 3.
In this manner, where the resin layer 2 is present between the tip end of the carbon nanotube 1 and the surface of the heat spreader 3, the thermal resistance in the portion is increased. As a result, the high thermal conductivity of the carbon nanotube 1 cannot be fully utilized. This leads to a problem in that the thermal conductivity between a semiconductor element and the heat spreader 3 is lowered.
On the other hand, where the amount of pressing is increased and the carbon nanotubes 1 are excessively pressed when fixed to the heat spreader 3, the carbon nanotubes 1 are laid down as shown in FIG. 5B. Consequently, it is highly likely that the elasticity (in this case, characteristic to recover to the original vertical arrangement) of the carbon nanotubes 1 is not fully utilized. In this case, the carbon nanotubes 1 cannot follow a deformation (warpage) of the semiconductor element occurring due to the heat generation. In this case too, the extremely thin resin layer 2 is highly likely to be interposed between the tip ends of the carbon nanotubes 1 and the surface of the heat spreader 3.